OK officially from a support ticket they made the wordl boss more resistant to stunning due to all the new heroes that can stun.

And I have critted since the patch with 141 and higher crit

You can stun on your first attack on any world boss, I do it all the time even after the patch

The bigger the banish the more resistant they are to stuns so a 1.25b banish is harder to stun then a 250m banish

FYI while the old setups for banish still work decently.

There are new combos to try out there that work better now with the new patch then the old forms did before the patch.

The patch as allowed me to increase the damage i do by about 40-50%